If you are unable to adjust the height of your table, you can raise or lower your chair, but this may cause an issue with the angle of your knees that will require more intervention such as a footrest or platform for your sewing machine. The reason the table height is important is that the goal is to be able to position your body so that the work surface and your elbows are on the same level. If your elbows are higher than your table, you will tend to hunch forward over the table. If your elbows are below your workspace, you will compensate by raising your shoulders to be able to fully reach your machine. Either way, you will end up with a sore neck, shoulders, or back. Or, all three.
If you already have a sewing workspace, one thing you can do to optimize it is to purchase a tiltable platform for your sewing machine. One nice feature of this platform is that it doesn't take up much space and is portable so you can use it anywhere you are able to sew.
Another thing to remember is to not rest your arms on the edge of your table, especially if they are sharp. This can lead to pinched nerves or compromised blood circulation. At the very least, you will end up with unsightly gouges in your arms when you are done sewing.
I hope you have found this information to be helpful. The next installment will discuss how to make your cutting surface more posture friendly as well as some neat tools that can make cutting and sewing easier.
Happy (and hopefully, pain-free) stitching!